Bearing unit, drainage pump and hydraulic turbine each incorporating the bearing unit

ABSTRACT

Disclosed is a bearing unit which can be used in a drainage pump operated without supply of clean water or a hydraulic pump, which exhibits excellent wear resistance against water containing earth and sand and satisfactory assembling facility, and a method of manufacturing the bearing unit. 
     The contact surface of a bearing made of stainless steel and/or that of a sleeve is applied with a sprayed coating, the main component of which is WC, and which contains one or more elements selected from a group consisting of nickel, chromium and cobalt as a binder material thereof, or a sprayed coating, the main component of which is Cr 3  C 2 , and which contains NiCr as a binder material, after the sprayed coating has been formed, heating at from 300° C. to 550° C. is performed for one hour or longer so that hardness and wear resistance equivalent to those of a WC - 12% sintered article is attained.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a drainage pump and, a hydraulicturbine, and more particularly, to a bearing unit exhibiting excellentwear resistance, satisfactory reliability and assembling facility, withthe bearing unit being adapted to be utilized in a pump of a typeadapted to be operated absent a clear water supply to the bearingportion thereof or a hydraulic turbine, and relates to a drainage pumpand a hydraulic turbine each incorporating the bearing unit as well as amethod for manufacturing the bearing unit.

2. Description of Related Art

Since substantially all precipitation flows in the drainage ditches dueto the recent urbanization, the precipitation cannot be completelydrained off and there is a tendency for municipal floods causing roadsto be under water. Hence, drainage facilities, each provided with adrainage pump system, have been established, causing another problemthat the cost of operation and maintenance increases. Therefore, anautomatic operation drainage pump system has been examined. The drainagepump system of this type must improve the performance and reliabilitythereof and non-water-supply operation technology. In order to achievethis, an increase in capacity and precedent-standby operation have beenstudied.

The non-water-supply operation technology is to operate the drain pumpwhile supplying no water to the space between a sleeve on a shaft and abearing. Since no clear water supply apparatus is provided, amalfunction does not take place due to a failure of the clear watersupply apparatus and that of a clear water supply sensor, resulting insatisfactory reliability. In order to enable the non-water-supplyoperation to be performed, a bearing unit must not be worn even if earthand sand contained in the drain invade thereto.

A conventional bearing unit adapted to the non-water-supply technologyhas been constituted by a sleeve (a sintered article) made of tungstencarbide (hereinafter "WC") exhibiting excellent wear resistance andcorrosion resistance and by a ceramic bearing (a sintered article).Constructions fashioned of a sleeve made of WC and the ceramic bearinghave been disclosed in JP-A-60-81517 and JP-A-60-88215.

The enlargement of the pump capacity inevitably requires the diameter ofthe bearing thereof to be enlarged. However, the conventional bearingunit fashioned of a WC sleeve and the ceramic bearing require alarge-diameter sintered sleeve and a sintered bearing. However, aproblem arises in that the sintering technology cannot meet therequirements and the weight of the parts cannot be reduced, resulting inthe assembling of the bearing unit. Therefore, a hard film coatingtechnology capable of coating a film having hardness equivalent to thatof ceramics has been investigated. It is noted that a non-water supplydrainage pump using a sleeve applied with a sprayed WC coating and aceramic bearing has been disclosed, for example, in "Trybologist", No.2, vol. 36, pp. 144 to 147.

Also the bearing for a hydraulic turbine must have wear resistanceagainst water including sediment. Therefore, a conventional bearing fora hydraulic turbine has been made of ceramics such as silicon carbide(SiC).

As a general rule, a sprayed coating is used as the hard film for thebearing portion of a drain pump. However, the fact that the sprayedcoating is inferior to a sintered article having the same composition interms of the hardness and the strength causes an investigation forimproving the hardness and the strength of the sprayed coating to bemade. For example, technology for improving the adhesive strength andthe strength by plasma-spraying a material composed of, by weight, 50%chromium and 50% nickel and by holding the sprayed coating at from about700° C. to 800° C. for from about 1 to 100 hours has been disclosed inJP-A-57-2872. Another technology has been disclosed in JP-A-60-149762which is capable of strengthening the sprayed coating by forming a 20 to80 wt. % Ni-Cr sprayed coating, the main component of which is Ni-P on aferrous material, and by subjecting them to a heat treatment set to fromabout 600° C. to 1000° C.

Since the hydraulic turbine has a large diameter, the conventionalstructure has an arrangement that sectioned ceramics are disposed.Therefore, there has been a desire of using the hard film similarly tothe bearing for a drain pump. A bearing unit for a hydraulic turbinethat uses a sprayed WC coating and chromium carbide (Cr₃ C₂) has beendisclosed on pp. from 80 to 85, in resumes of the twenty-sixth lecturemeeting titled "Turbo Machines", May 1991.

The precedent standby operation is an idle operation executed beforecommencement of drainage in a state where the water level is lower thanthe level at which the drainage operation can be performed. Byperforming the precedent standby operation, full power operation can beperformed when the water level rises to a level at which the drainagecan be performed. Therefore, a rapid increase in precipitation can bequickly accommodated with the subject system. However, the fact that thebearing must be idly performed without drain to be introduced into thesliding or bearing portion thereof for a short time causes the frictioncoefficient between the bearing and the sleeve must be extremely low soas to be freed from being damaged during the foregoing dry operation.

A bearing capable of performing both of a long time precedent standbyoperation, in which the idle operation is carried out for a long time,and the non-water-supply operation has not been developed yet. However,a method capable of supplying external water to the sliding portion ofthe bearing at the time of the idle operation has been proposed inJP-A-55-90718.

In order to drain water containing earth and sand, the bearing must havesatisfactory wear resistance. In particular, the non-water supplybearing must have wear resistance against slurry because clear water forprotecting the bearing and the sleeves is not supplied. The majorportion of the earth and sand is composed of feldspar and quartz.Therefore, earth and sand have a maximum hardness of about Vickers(Hv)1000 (which is the hardness of quartz). Accordingly, the hardnessrequired for the bearing and the sleeve is Hv 1000 or higher.

A sintered article composed one of WC, SiC and silicon nitride (Si₃ N₄)is excellent in the hardness while providing satisfactory wearresistance because these materials have a hardness of Hv 1000 or higher,that is, WC has a hardness of about Hv 1400, SiC has a hardness of aboutHv 2800 and Si3N4 has a hardness of about Hv 1600. However, thesintering technology, the assembling facility and the manufacturing costcause a limit to present in the size of the sleeve and the bearing thatare fully made of a sintered material composed of WC, SiC or Si₃ N₄.Therefore, there has been investigated the use of a hard film in placeof the aforesaid sintered article. Since the film thickness must beseveral hundreds microns if the quantity of wear is taken intoconsideration, there is a limit in the method of manufacturing the hardfilm. As the hard film for the bearing portion of a drainage pump, athermally-sprayed coating mainly composed of WC or Cr₃ C₂ and having athickness of from about 100 to 200 μm has been widely used because arelatively thick film and satisfactory hardness can be obtained.However, the sprayed coating mainly composed of WC or Cr₃ C₂ generallyhas unsatisfactory hardness as compared with a sintered articlealthough, depending upon the material, the forming method andconditions, with the aforesaid sprayed coating has a hardness of fromabout 600 Hv to 1000.

The relationship between a variety of conventional sprayed coatings andthe wear resistance against water containing earth and sand will bedescribed with reference to FIG. 21 which is a graph showing the wearrates measured by element experiments to be described later in thedescription about preferred embodiments. Each one of the sprayedcoatings were formed on a stainless steel plate to serve as a rotaryside specimen, while α- SiC was employed to serve as the stationary sidespecimen. The residual conditions were as follows: the surface pressurewas 2 kg/cm², the peripheral speed was 0.5 m/sec, the concentration ofsilica particles in earth and sand in water was 9 wt. %. It is notedthat the detailed shape of the specimen and the sliding method and thelike were the same as those of the experimental conditions shown in FIG.19 with regard to the preferred embodiments.

The abscissa of a graph shown in FIG. 21 stands for cross-sectionalVickers hardness of the sprayed coatings, and the ordinate stands forrelative wear ratio standardized while making the wear ratio of WC - 12%cobalt sintered article to be a reference. Sprayed coatings 1, 2 and 3were conventional sprayed coatings formed by a high speed sprayingmethod and a detonation spraying method and mainly composed of WC or Cr₃C₂. As shown in FIG. 21, the wear resistance against water containingearth and sand considerably depends upon the hardness of the sprayedcoating. It is preferred that a satisfactory effect can be obtained ifthe hardness is Hv 1000 or higher as described above. As can beunderstood from from FIG. 21, the conventional sprayed coating cannotprovide satisfactory hardness and wear resistance against watercontaining earth and sand.

In general, spraying is an operation in which not only hard particlessuch as WC or Cr₃ C₂ are heated and sprayed but also metal particlessuch as nickel, chromium or cobalt particles are mixed andsimultaneously heated and sprayed. As a result, WC particles or Cr₃ C₂particles are bound by molten metal of nickel, chromium or cobalt toform a film. The reason why the sprayed coating is lower in hardnessthan that of the sintered article is not that the WC or Cr₃ C₂ particlesare not of a lower hardness but that the binder metal for binding the WCor Cr₃ C₂ particles has detects such as blow holes or that the bindingstrength between the binder metal and the WC or Cr₃ C₂ particles is notfully high.

In order to enlarge the binding strength between WC or Cr₃ C₂ particles,the spraying method has been improved variously. For example, a highspeed flame spraying method has been proposed for increasing thespraying velocity of the particles by utilizing the combustion energy ofa combustible gas and a detonation spraying method utilizing adetonation of a combustible gas. As a result, the particle velocity atthe time of spraying can be increased as compared with the speedrealized when the conventional plasma sprayed coating is formed,resulting in a sprayed coating of higher hardness. However, the wearresistance against sediment water containing earth and sand isinsufficient even if the sprayed coating is formed by any of theaforesaid spraying methods. It has been understood from the element testshown in FIG. 19 that the reason for this resides in the fact thehardness of the sprayed coating is lower.

The foregoing conventional ceramic bearings and those disclosed inJP-A-60-81517 and JP-A-60-88215 are not technologies that have beensuccessful when taking into consideration the difficulty in thesintering technology, deterioration in the reliability andunsatisfactory assembling facility due to an increase in the weightcaused from increasing the diameter of the bearing and the sleeve.

The non-water supply drainage pump using a combination of the WC sprayedcoating and the ceramic bearing (sintered article) ("Trybologist", No.2, vol. 36) and the bearing structure for a hydraulic turbine that usesa WC sprayed coating and Cr₃ C₂ sprayed coating (resumes of thetwenty-sixth lecture meeting titled "Turbo Machines") are notsuccessfully while considering the hardness factor, the wear resistanceand the reliability of the sprayed coating, with the main componentbeing WC or Cr₃ C₂.

The proposed improved methods for properties of the sprayed coatingaccording to, for example, JP-A-57-2872 and JP-A-60-149762, encounter alimitation in coating materials for use therein. As the coatingmaterial, those having a thermal expansion coefficient whichapproximates that of ferrous alloys, which are primary base materials,or these which are not decomposed by the improving heat treatment, areused. The sprayed coating, the main component of which is WC or Cr₃ C₂,and from which satisfactory hardness can be attained, results in a largedifference in the thermal expansion with respect to the base material.As a result, the coating is ruptured and it cannot be used as it is.Furthermore, an apparatus such as a pump that is used in water must bemainly made of stainless steel. Therefore, according to the conventionalheat treatment of improving properties of the sprayed coating, the heattreatment temperature is higher than the annealing temperature of astainless steel as a base material or the temperature at which thestainless steel is sensitized to intergranular corrosion. Hence, thehardness and the wear resistance of the base material deteriorate,causing a problem to arise in that the reliability deteriorates.

That is, the prior art has not been taken into consideration anyimprovement in properties of the sprayed coating, the main component ofwhich is WC that has a large difference in the thermal expansioncoefficient from ferrous alloys. Moreover, no consideration has beengiven with regard to the hardness of the base material that wearresistance.

The prior art disclosed in JP-A-55-90718 and having the arrangement thatexternal water is supplied to the sliding portion of the bearing resultsin a problem of reliability because the operation of the drainagefacility is stopped due to a malfunction of an external water supplyapparatus and the drainage operation can be stopped in an emergency.

SUMMARY OF THE INVENTION

An object of the present invention resides in providing a bearing unitadaptable to a drainage pump system or a hydraulic turbine, exhibitingexcellent wear resistance and assembling facility and satisfactoryreliability, and to a drainage pump and a hydraulic turbine eachincorporating the bearing unit, and a method of manufacturing thebearing unit.

The object of the present invention can be achieved by improving thewear resistance of a sprayed coating applied to the bearing surface of abearing unit by heating the sprayed coating, the main component of whichis WC or Cr_(3C) ₂, while determining the temperature conditions so asto make the sprayed coating to be freed from deterioration of theadhesive property and denaturing, and make the base material ofstainless steel to be freed from excessive deterioration of the hardnessand sensitization to intergranular corrosion due to thermal stress.

A bearing unit of the present invention comprising a bearing and asleeve which is carried by the bearing has an arrangement that thecontact surface of the bearing and/or that of the sleeve is applied withthe following sprayed coating: a sprayed coating, the main component ofwhich is WC, and which contains one or more elements selected from agroup consisting of nickel, chromium and cobalt as a binder materialthereof; or a sprayed coating, the main component of which is Cr₃ C₂,and which contains nickel and chromium as binder materials thereof. Thenumber of blow holes having a size of 20 μm or larger formed in thesurface of the sprayed coating is fifteen per one square millimeter(mm²) or less.

The formed sprayed coating is heated at a temperature from 300° C. to550° C. for not less than one hour to have a hardness of not less thanHv 1000 to have sufficient wear resistance against sediment watercontaining earth and sand.

The material of the bearing and/or the sleeve applied with the sprayedcoating must be ferrous alloys, preferably stainless steel.

If either of the bearing and the sleeve is applied with the sprayedcoating, the residual is of SiC or Si₃ N₄ provided at the contact orbearing surface thereof or may be entirely made of SiC or Si₃ N₄.

The sleeve or the bearing may be divided into two or more sections.

The drainage pump or the hydraulic turbine according to the presentinvention incorporates a bearing unit characterized by anyone of thearrangements stated above.

A bearing and/or a sleeve of the present invention are manufactured bythe following steps: a sprayed coating, the main component of which isWC, and which contains one or more elements selected from a groupconsisting of nickel, chromium and cobalt as a binder material thereof,or a sprayed coating, the main component of which is Cr₃ C₂, and whichcontains nickel and chromium as binder materials thereof, is formed onthe contact surface of the bearing and/or the sleeve machined to have apredetermined shape and made of stainless steel; the bearing and/or thesleeve is heat treated at from 300° C. to 550° C. for not shorter thanone hour; and it is subjected to finish machining as to have apredetermined size.

As noted above, upon heating of a sprayed coating, the primary componentof which is WC or Cr₃ C₂, will release the strain which contributes toharden the coating, and therefore the hardness is lowered. It has beenconsidered that, if the spraying coating mainly composed of WC or Cr₃ C₂having a small thermal expansion coefficient is formed on a base made ofa ferrous alloy, for example, a stainless steel of JIS SUS403, heatingof the sprayed coating after the coating has been formed causes thefollowing problems due to the difference in the thermal expansioncoefficient; namely, adhesion strength of the coating deteriorates dueto the thermal stress, and WC particles or Cr₃ C₂ particles areoxidized, causing the properties of the sprayed coating to deteriorate.Therefore, it has been considered that it is not preferable to heat thesprayed coating mainly composed of WC or Cr₃ C₂ after the coating hasbeen formed. However, the hardness can be improved while eliminatingadverse influence on the sprayed coating if proper heating conditionsare selected. For example, the hardness of the sprayed coating can beimproved by heating from 300° C. to 550° C. for not less than one hourto have sufficient wear resistance against water containing earth andsand.

If blow holes are present in the sprayed coating, the strength of thesprayed coating is decreased. Furthermore, the binding strength betweenparticles of WC or Cr₃ C₂ is also decreased, causing the particles of WCor Cr₃ C₂ to be separated from the coating during operation. As aresult, the wear resistance deteriorates. However, heat treatment of theformed coating performed for the purpose of eliminating the blow holeswill cause particles of the binder metal, and particles of WC or Cr₃ C₂to be oxidized, adhesion strength of the sprayed coating to deteriorate,and the hardness and the corrosion resistance of the base material todeteriorate. As a result, the reliability deteriorates. However, it hasbeen found that the present invention enables the number of the blowholes in the sprayed coating to be decreased, with the size of the blowhole to be reduced and the hardness of the sprayed coating to beimproved by a heat treatment at a low temperature which does not causethe oxidation of the WC or Cr₃ C₂ sprayed coating, deterioration ofadhesion strength of the coating due to the thermal stress and thedeterioration of the hardness and the corrosion resistance of the basematerial. The above effects can be obtained by selecting the temperatureand the time in which heating is performed.

In particular, it has been found that heat treatment conditionsaccording to the present invention enables the number of large blowholes to be decreased, herein, the large blow holes being defined asthose having a size of not smaller than 20 μm that is larger than themaximum particle size of particles of WC or Cr₃ C₂. The large blow holesseparate particles of WC or Cr₃ C₂ from the coating. Therefore, thedecrease in the number of the large blow holes may improve the strengthof the sprayed coating. If the number of the large blow holes aredecreased, the sprayed coating is attained ductility, causing resistanceagainst impact to be improved. As a result, occurrence of rupture andseparation of the coating can be prevented and the reliability of thecoating therefore is improved.

Since the blow hole serves as the trigger point of corrosion of thesprayed coating, the decrease in the number of the blow holes causesparticles of WC or Cr₃ C₂ to be positioned more closely to one another.Therefore, the corrosion generated among particles can be prevented andthe corrosion resistance against drain can be improved.

If the drainage pump of the present invention is used to serve as aprecedent standby operation drainage pump, the arrangement that thebearing, which can be operated without lubrication, is employed willenable satisfactory reliability to be attained.

By constituting the sleeve and/or the bearing to have a separatedstructure, the manufacturing process can be simplified. It is especiallysuitable to a large-diameter pump.

Other and further objects, features and advantages of the invention willbe appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view which illustrates the structure of adrainage pump according to an embodiment of the present invention;

FIG. 2 is a perspective view partially including a cross sectional viewwhich illustrates a bearing unit according to an embodiment of thepresent invention;

FIG. 3 is a vertical cross sectional view which illustrates a bearingunit according to another embodiment of the present invention;

FIG. 4 is a vertical cross sectional view which illustrates a bearingunit according to another embodiment of the present invention;

FIG. 5 is a vertical cross sectional view which illustrates a bearingunit according to another embodiment of the present invention;

FIG. 6 is a perspective view which illustrates the structure of a sleeveaccording to another embodiment of the present invention;

FIG. 7 is a cross sectional view taken along line VII--VII of FIG. 6;

FIG. 8 is a perspective view partially including a cross sectional viewwhich illustrates a bearing according to another embodiment of thepresent invention;

FIG. 9 is a graph which shows the relationship W/D² between the outerdiameter "D" of a sleeve according to another embodiment of the presentinvention and the weight "W";

FIG. 10 is a cross sectional view which illustrates the structure of ahydraulic turbine according to another embodiment of the presentinvention;

FIG. 11 is a graph which illustrates change in the hardness of a WC -27% NiCr sprayed coating taken place when it is heated at 400° C.;

FIG. 12 is a graph which illustrates change in the hardness of a WC -27% NiCr sprayed coating taken place when it is heated at 500° C.;

FIG. 13 is a graph which illustrates change in the hardness of a WC -12% cobalt sprayed coating taken place when it is heated at 400° C.;

FIG. 14 is a graph which illustrates change in the hardness of a Cr₃C₂ - 25% NiCr sprayed coating taken place when it is heated at 400° C.;

FIGS. 15A, 15B and 15C are photographs which illustrate the change ofthe surface of the WC - 27% NiCr sprayed coating when it is heated at400° C.;

FIG. 16 is a graph which illustrates the relationship between the numberratio of blow holes in the surface of the WC - 27% NiCr sprayed coatingand the heating time when it is heated at 400° C.;

FIG. 17 is a graph which illustrates the relationship between the numberratio of blow holes in the surface of the WC - 27% NiCr sprayed coatingand the hardness when it is heated at 400° C.;

FIGS. 18A, 18B, 18C and 18D illustrate the shape of the wear testspecimen and the method of sliding it;

FIG. 19 illustrates the relationship between the distance of sliding andthe wear loss obtained by a wearing test subjected to a WC - 12% cobaltsintered article, the WC - 27% NiCr sprayed coating subjected to no heattreatment and the WC - 27% NiCr sprayed coating subjected to the heattreatment;

FIG. 20 is a graph which illustrates the relative wear rates ofcombinations of materials including the WC - 27% NiCr sprayed coatingsubjected to the heat treatment, Cr₃ C₂ - 25% NiCr subjected to the heattreatment; and

FIG. 21 is a graph which illustrates the hardness of conventionalsprayed coatings and relative wear ratio against water containing earthand sand with respect to WC - 12% sintered article as a referencematerial.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

In general, a drainage pump has two bearings respectively disposedadjacent to an impeller and in an upper portion thereof. A bearing unitaccording to this embodiment can be adapted to either of the twobearings. Referring to FIG. 1, reference numeral 1 denotes a main shaft,2 a sleeve fixedly attached on the main shaft 1, and 3 a bearing. Themain shaft 1, the sleeve 2 and the bearing 3 are fashioned of astainless steel of JIS SUS403.

The pump is provided with a plurality of pipes 30 each of which has alower end opening in a drain suction port zone of a pump casing and anupper end opening in the atmosphere, and is vertically arranged alongthe outer surface of the casing, and which is supported by a rib of thecasing at a lower end and by a floor of the drainage facility.

In the pump, if the drainage water level is low, difference takes placebetween the internal pressure of the pump and the atmosphere, causingair to be sucked into the drain through the pipe.

As a result, the quantity of the flow is reduced and generation ofvortexes in the drain level is restricted. The generation of thevortexes must be prevented because it causes vibration force to begenerated, thus resulting in the vibration of the pump. By sucking airthrough the pipe 30, the generation of the vortexes can be preventedeven if the water is a low level at which the vortexes are easilygenerated. Therefore, drainage can be performed stably.

If the water level has been raised sufficiently, the pressure differenceacross the casing is reduced, causing the suction of air through thepipes 30 to be interrupted. Since the pressure change in the casingtaking place due to the drain level is utilized, any external control isnot required and accordingly satisfactory reliability is attained.

By providing a valve at the upper opening end portion of the pipes 30,the quantity to be suctioned can be controlled and the above-mentionedfunction can be cancelled.

As shown in FIGS. 2 and 3, the sleeve and the bearing 3 are manufacturedas follows: a sleeve base 8 and a bearing base 10 are manufactured bymachining; and heat treatments of quenching and tempering are performedto improve the hardness of a stainless steel of JIS SUS403. Thetempering temperature is about 700° C. so that the stainless steel ofJIS SUS403 does not become brittle at the time of improving the propertyof the sprayed coating to be performed later. After the heat treatmentwas completed, sprayed coatings 11 and 9 were formed on a slidingportion of the bearing 3 and the sleeve base 8 by a high speed flamespraying method, the sprayed coatings 11 and 9 being mainly composed ofWC and containing NiCr as a binder metal. After the sprayed coatings 9and 11 were formed, a heat treatment was performed by holding thebearing 3 and the sleeve base 8 at 400° C. for twenty hours to improvethe hardness of the sprayed coatings 9 and 11. After the heat treatmentwas completed, the sleeve 2 and the bearing 3 were machined torespectively have predetermined inner and outer diameters and apredetermined surface roughness. The sleeve 2 is secured to the mainshaft 1 by a rotation stopper 4.

The bearing 3 is fastened to a metallic backing member 6 for the bearing3 followed by being fixed by a fixing member 7 while interposing abuffer material 5 for the bearing 3. Furthermore, an invasion preventionmember 12 for preventing invasion of sediment into the sliding portionwas fastened to the bearing 3.

Since this embodiment has the arrangement that stainless steel of JISSUS403 is employed as the material for the sleeve base 8, its thermalexpansion coefficient coincides with that of the material of the mainshaft 1. As a result, generation of thermal stress can be prevented.Furthermore, the realized satisfactory toughness enables the sleeve tobe obtained which is able to stably holding the sprayed coating formedon the surface thereof.

Furthermore, the heat treatment applied to the WC sprayed coatingenables the wear resistance to be improved, causing the durability andthe reliability of the bearing unit to be improved.

Although this embodiment has the arrangement that the sprayed coating tobe formed on the sleeve and the bearing is mainly composed of WC andcontains NiCr as the binder material, a similar effect was obtained froma sprayed coating, the main component of which was WC, and which usedcobalt as the binder material, and a sprayed coating, the main componentof which was Cr₃ C₂, and which used NiCr as the binder material.

The drainage pump exhibits stable bearing performance even if thebearing unit is under non-lubrication state, that is, non-water-supplystate. If water is supplied, the drainage pump is able to furthersatisfactorily perform the bearing operation.

The drainage pump has also an invasion prevention member to preventinvasion of sediment into the bearing or contact portion of the bearingunit. If sediment mixed with the drain, however, invades into thesliding portion, the operation can be performed stably because thebearing portion exhibits satisfactory wear resistance.

Since excellent bearing performance is attained in a non-lubricationstate, the drainage pump can be reliably operated in a state whereno-lubrication operation and lubricated operation are repeated, that is,in the precedent standby operation.

As shown in FIG. 4, the contact surface of the sleeve base 8, made of astainless steel of JIS SUS403, is applied with the sprayed coating 9,the main component of which was WC, and which contained cobalt as thebinder material. Furthermore, a bearing 13 which supports the sleeve 2,is made of SiC. The sleeve 2 was subjected to a heat treatment of 400°C. for twenty hours after the sprayed coating 9 was formed so as toimprove the hardness of the sprayed coating 9. The residual structuresare the same as those described above in connection with FIGS. 1 to 3.

Since this embodiment has an arrangement that the material of thebearing portion is uniform, hard and sintered ceramics, the durabilityand the reliability of the bearing unit can be improved.

Although this embodiment has the arrangement that the sprayed coating tobe formed on the sleeve is mainly composed of WC and contains NiCr asthe binder metal, a similar effect is obtained from a sprayed coating,the main component of which is WC, and which uses cobalt as the bindermaterial, and a sprayed coating, the main component of which was Cr₃ C₂,and which used NiCr as the binder material.

The embodiment of FIG. 5 illustrates an arrangement about a bearingstructure capable of performing an idling operation even if the drainagewater level does not reach the height of the bearing at the time of aprecedent standby operation. It is noted that the detailed crosssectional structure of the sleeve 2 is omitted from illustration inorder to show the overall structure. A sprayed coating 11, the maincomponent of which is WC, and which contained NiCr as the binder metal,is applied to the contact surface of the sleeve 2 made of stainlesssteel of JIS SUS403. After the sprayed coating 11 has been formed, aheat treatment set to 400° C. for 20 hours is performed to improve thehardness of the sprayed coating. Furthermore, the bearing 13 whichsupports the sleeve 2 is made of SiC. The bearing 13 is fastened to themetallic backing member 6 and fixed to a fixing member 7' whileinterposing the buffer material 5. Furthermore, a backing plate 17 issecured to the fixing member 7'In addition, a rotary water tank 18 issecured to the main shaft 1. Water is enclosed in the rotary water tank18 so that lubricant water is present in the contact portion of thebearing if the idle operation is performed in which the drainage waterlevel does not reach the height of the bearing. The structure accordingto this embodiment encounters a fact that earth and sand are sometimesinvaded into the contact portion at the time of the operation becausethe rotary water tank 18 easily accumulates the sediment. However, thesprayed coating 11 on the contact surface of the sleeve 2 has sufficientwear resistance attained due to the heat treatment, resulting no problemto arise.

In accordance with the embodiment of FIGS. 6 and 7, fourcircumferentially disposed sleeve sections 2a, 2b, 2c and 2d ofstainless steel of JIS SUS403 are provided. A sprayed coating, the maincomponent of which is Cr₃ C₂, and which contained NiCr as the bindermetal, is formed on the contact surface side of each of the sleevesections 2a, 2b, 2c and 2d with which the bearing is brought intocontact. The sleeve sections 2a, 2b, 2c and 2d were subjected to a heattreatment of 400° C. for twenty hours. Each of the sleeve sections 2a,2b, 2c and 2d is fixed to the main shaft 1 by two bolts 14 at the axialdirectional two portions thereof. The residual structures are the sameas those according to the foregoing embodiments shown in FIGS. 1 to 3.

The sleeve sections 2a, 2b, 2c and 2d are manufactured by usingstainless steel of JIS SUS403, and they are subjected to the heattreatment arranged similarly to that applied to the method ofmanufacturing the bearing. After the heat treatment has been completed,sprayed coating WC, the main component of which is WC, and whichcontains NiCr or cobalt as the binder material, is formed on the contactsurface of each sleeve by high speed flame spraying. After the sprayedcoating has been formed, the sleeve, formed by the sleeve sections 2a,2b, 2c, 2d is heated to 400° C. and held at the temperature for twentyhours. After the heat treatment has been completed, the sleeve is fixedto the main shaft 1, and the sleeve is machined to have a predeterminedsize. Although the bolts are sufficient to fix the sleeve to the mainshaft 1, use of an adhesive agent will improve the reliability.

Since this embodiment has an arrangement that the sleeve is divided intoa plurality of sections 2a, 2b, 2c, 2d, the operation for forming thesprayed coating can be facilitated. Furthermore, the width of each ofthe divided sleeve sections can be reduced to a quarter of thecorresponding integrated sleeve, causing an effect to be obtained inthat the sleeve can be handled easily.

In the embodiment of FIG. 8 an arrangement is provided wherein bearingsections 19 are formed by dividing the bearing portion of a bearing.Each of the bearing sections 19 is produced by forming the sprayedcoating 11 thereon, the main component of which is WC, and whichcontains NiCr or cobalt as the binder metal, by high speed flamespraying on the base 10 made of stainless steel of JIS SUS403. After thesprayed coating has been formed, a heat treatment of 400° C. for twentyhours is performed to improve the hardness of the sprayed coating. Thebearing sections 19 are fastened to the metallic backing member 6 whileinterposing the buffer material 5, the bearing sections 19 being thendisposed in a bearing housing 20. Although the bearing sections 19 arefastened to the bearing housing 20 by a fixing jig 21, use of anadhesive agent will improve the reliability.

Since this embodiment has an arrangement that the bearing portion of thebearing is divided into a plurality of sections, the sprayed coating caneasily be formed. Furthermore, the arrangement that each sliding section19 is fixed while interposing the buffer material 5 will effectivelyprevent an unsymmetrical contact.

The sleeve which has been made by sintered carbide mainly composed of WCcan be made of a stainless steel according to the present invention.Therefore, the weight can be considerably reduced. Furthermore, thearrangement that the sleeve base is made of stainless steel will enablethread hole machining to be performed. Therefore, handling can befacilitated. In particular, a significant effect can be obtained fromthe present invention if a sleeve has a large diameter. The effects ofthe present invention attained to reduce the weights of sleeves arecollectively shown in FIG. 9. Since the sleeve weight "W" is varied whenthe outer diameter "D" of the sleeve is changed, the relationship isstandardized to W/D². According to the present invention a W/D² of 0.05or less is attained.

Referring to FIG. 10, reference numeral 22 represents a main shaft for ahydraulic turbine, and 23 represents a sleeve rotation stopper.Reference numeral 24 represents a sleeve made of stainless steel of JISSUS403 manufactured by forming a sprayed coating (hereinafter called a"WC - 27% NiCr sprayed coating"), the main component of which is WC, andto which NiCr is added by 27 weight % as a binder metal, followed byheat treatment applied thereto. Reference numeral 25 represents abearing made of stainless steel of JIS SUS403 manufactured by forming asprayed coating (hereinafter called a Cr₃ C₂ - 25% NiCr sprayedcoating), the main component of which is Cr₃ C₂ subjected to a heattreatment, and to which NiCr is added as a binder metal by 25% and byapplying heat treatment. Reference numeral 26 represents a liner. Thesprayed coating was formed on the surface of the sleeve and the bearingby using a high speed flame spraying method. As can be understood fromthe aforesaid results of the element test, the sprayed coating subjectedto the heat treatment has wear resistance similar to that attained fromthe sintered article. Furthermore, the aperture can easily be enlargedand therefore an effect of reducing the cost and the weight can beobtained if the arrangement of this embodiment is adapted to a hydraulicturbine.

The wear resistance properties of the bearing according to the foregoingembodiment were evaluated by examining the change in the hardness of thesprayed coating taken place due to heat treatment applied. The change inthe hardness of the sprayed coating was examined under conditions:heating temperature was ranged from 200° C. to 600° C.; and heating timewas ranged from 1 to 30 hours. The hardness of the sprayed coating wasmeasured by a Vickers hardness meter. In order to eliminate theinfluence of the base, the hardness was measured at the cross sectionalof the sprayed coating and a load of 300 g is equally applied.

FIG. 11 illustrates a portion of the results of aforesaid measurements,wherein change in the hardness taken place when a WC - 27% NiCr sprayedfilm is heated to 400° C. is shown. The abscissa stands for the heatingtime, while the ordinate stands for the hardness of the sprayed coating.The sprayed coating was formed by high speed flame spraying to have athickness of about 100 mm after the surface has been polished. The baseis made of stainless steel of JIS SUS403 and is subjected to heattreatments of quenching and tempering at about 700° C. Since the valuesof the measured hardness disperse, ten points were measured to show themaximum value and the minimum value of the measured values aredesignated by a line while showing average values by a mark "◯".

As shown in FIG. 11, the hardness of the sprayed coating increases withheat treatment time and reaches its maximum hardness in ten to thirtyhours. Then, the hardness becomes substantially constant value althoughslight dispersion takes place. The maximum value of the average hardnessis Hv 1014 (maximum value Hv 1065 and minimum value Hv 890). The thusattained hardness is inferior to an average value of Hv 1317 (maximumvalue Hv 1404 and minimum value Hv 1235) of the hardness of a sinteredarticle (hereinafter called a "WC - 12% cobalt sintered article"), themain component of which is WC, and to which cobalt is, as the binder,added by 12 weight %. However, an increase of an average value of about34% is attained as compared with an average value of Hv 727 (maximumvalue Hv 869 and minimum value Hv 604) of the hardness realized beforethe heat treatment is performed, resulting in a significant improvement.

FIG. 12 illustrates a part of the results of the measurements, whereinchange in the hardness taken place when a WC - 27% NiCr sprayed coatingis heated to 500° C. is shown similarly to FIG. 11. The method offorming the sample, the shape of the same and the method of measuringthe hardness are the same as those employed to obtain the results shownin FIG. 11. The hardness of the sprayed coating substantially reachesthe maximum hardness when the sample is heated for about 1.5 hours. Themaximum value of the average hardness is Hv 1115 (maximum value Hv 1139and minimum value Hv 1084). The average value is increased by about 47%as compared with the hardness realized before the heat treatment isapplied, resulting in similar hardness to that of the WC - 12% cobaltsintered article.

Also FIG. 13 shows a part of the results of the measurements, whereinchange in the hardness taken place when a sprayed coating (hereinaftercalled a WC - 12% cobalt sprayed coating), the main component of whichis WC, and to which cobalt is, as the binder material, added by 12weight %, is heated to 400° C. is shown. Also the WC - 12% cobaltsprayed coating is formed by the high flame spraying similarly to eachsample shown in FIGS. 11 and 12 to have a thickness of about 100 μmafter its surface has been polished. The base is made of stainless steelof JIS SUS403 subjected to the heat treatment. Also the shape of thesample and the method of measuring the hardness are the same as thoseemployed to obtain the results shown in FIGS. 11 and 12. The hardness ofthe WC - 12% cobalt sprayed coating increases with heat treatment timeand reaches the maximum hardness in 10 to 20 hours, followed by slightdeterioration in 25 hours. The average value of the maximum hardness isHv 946 (maximum value Hv 1149 and minimum value Hv 792). An increase inthe average value of about 62% is attained as compared with an averagehardness value of Hv 584 (maximum value Hv 1149 and minimum value Hv490) realized before the heat treatment. A similar hardness change tothat attained in the case of the WC - 27% NiCr sprayed coating was takenplace in the case where heating to 500° C. was performed. In this case,the maximum hardness was attained by performing heating for about onehour.

Also FIG. 14 shows a portion of the results of the measurements, whereinchange in the hardness is shown which takes place when a Cr₃ C₂ - 25%NiCr sprayed coating is heated at 400° C. Also the Cr₃ C₂ - 25% NiCrsprayed coating is formed by a similar high speed flame sprayingemployed to form each sample for use in the measurement shown in FIGS.11, 12 or 13 to have a thickness of about 100 μm after its surface hasbeen polished. The base is made of stainless steel of JIS SUS403subjected to the heat treatment. The shape of the sample and the methodof measuring the hardness are the same as those employed to obtain theresults shown in FIGS. 11, 12 and 13. The hardness of the Cr₃ C₂ - 25%NiCr sprayed coating gradually increases with heat treatment time. Themaximum value of the average hardness in 25 hours of heating is Hv 958(maximum value Hv 1043 and minimum value Hv 905). The average valueincreased by about 21% as compared with an average value Hv 791 (maximumvalue Hv 817 and minimum value Hv 744) of the hardness before the heattreatment is performed. In this case, the attained increased is inferiorto that realized in the case of the WC - 27% NiCr sprayed coating andthe WC - 12% cobalt spray coating. Also the hardness increases with timelikely the case in which heating to 400° C. is performed, resulting in alarger degree.

The hardness of each of the foregoing WC - 27% NiCr sprayed coating, theWC - 12% cobalt sprayed coating and the Cr₃ C₂ - 25% NiCr sprayedcoating is significantly improved due to the heating from 400° C. to500° C., resulting in hardness equivalent to the hardness of WC - 12%cobalt sintered article. It can be considered that the heating from 400°C. to 500° C. increases the binding strength for particles of WC or Cr₃C₂ with the binder material in the sprayed coating to a levelsubstantially equivalent to the sintered article.

The results of examinations of the heating temperature range from 200°C. to 600° C. and a heating time range from one to thirty hours will nowbe described. Although the hardness of the sprayed coating increases ifthe heating temperature is set to 300° C. or lower, the increase ratiois excessively low, resulting in a difficulty in industrial use. It istherefore preferable that the heat treatment temperature be 300° C. orhigher, more preferably 350° C. or higher. However, thermal stress isgenerated, causing the adhesive strength of the film to deteriorate andresulting in a separation of the film from the base material in anexcessive case due to the difference in the thermal expansioncoefficient between the sprayed coating and the base because of thefollowing fact: ferrous alloys for use to form the pump has a thermalexpansion coefficient of about 12 to 17×10⁻⁶ /°C. and WC or Cr₃ C₂ typesprayed coating has a thermal expansion coefficient of about 5 to 7×10⁻⁶/°C. Furthermore, the heat treatment performed in the atmosphere willcause an oxidation of the sprayed coating to take place, resulting in adeterioration of the wear resistance. Therefore, if the WC or Cr₃ C₂sprayed coating is used, the heat treatment temperature must be set to550° C. or lower, preferably 500° C. or lower. In order to prevent theoxidation of the sprayed coating, it is preferred to perform heating ininert gas or in vacuum.

FIG. 15 shows a microscopic photograph of a state where the WC - 27%NiCr sprayed film formed to have a thickness of abut 200 μm was heatedat 400° C. followed by performing surface polishing until the thicknessis reduced to about 100 μm, and heating is again performed followed byperforming polishing to attain a mirror surface. The final thickness isabout 90 μm. FIG. 15A is a photograph of the surface of non-treatedsprayed coating, FIG. 15B is a photograph of the surface of the sprayedcoating after it has been heated for 10 hours, FIG. 15C is a photographof the surface of the sprayed coating after it has been heated for 20hours. Each of the photographs are taken at a magnification of 100times. Since the observation is observed with no etching performed, thedifference between the WC particles and the binder material NiCr cannotbe observed, resulting in being observed as a full white base. Referringto FIGS. 15A, 15B and 15C, dotty black portions are blow holes.Comparing FIGS. 15A, 15B and 15C, the number of the blow holes decreasesthe size of the same is reduced with a lapse of the heat treatment time.Since the samples for use in the experiments are, by cutting, obtainedfrom a sprayed coating formed on a plate (26 mm×26 mm) made of stainlesssteel of JIS SUS403, the difference in the number of the blow holes hasnot taken place due to the spraying conditions but has taken place dueto the fact that the number of the blow holes has decreased by the heattreatment. A similar phenomenon was observed in the WC - 12% cobaltsprayed coating or the Cr₃ C₂ - 25% NiCr sprayed coating.

FIG. 16 illustrates the relationship between the heating time anddensity of blow holes having a size larger than 20 μm observed on thesurface when the WC - 27% NiCr sprayed coating is heated at 400° C. Itshould be noted that the method of observing the blow holes formed inthe surface is the same as the method employed to obtain the resultsshown in FIG. 15. When the structure of the WC - 27% NiCr sprayedcoating was observed by a scanning type electronic microscope, theaverage particle size of the WC particles and the particle of the NiCrbinder material was about 10 μm and the maximum size was 20 μm or less.Therefore, the presence of blow holes having a size of several μm doesnot affect the separation of the WC particles. If the size of the blowholes is larger than 20 μm, the WC particles are separated, causing thestrength of the sprayed coating to deteriorate. Therefore, the blowholes larger than the particle size of 20 μm or more were measured. Theterm "blow holes" means blow holes having a size of 20 μm hereinafter.

As shown in FIG. 16, the number of blow holes rapidly increases if thesprayed coating is heated at 400° C. for 15 hours or longer. The changein the number of the blow holes coincides with the change in thehardness shown in FIG. 7. That is, it can be considered that the changein the hardness of the sprayed coating due to heating takes place isdependence upon the number ratio of the blow holes in the sprayedcoating.

The relationship between the number of the blow holes and the hardnessobtained from the change in the hardness of the heating time and thechange in the number ratio of the blow holes respectively shown in FIGS.12 and 16 is shown in FIG. 17. If the number of the blow holes is 30 persquare milimeter (mm²) or lower, the hardness commences to increase, andthe hardness preferable for the bearing portion of the pump is realizedwhen the number is 15 per square milimeter (mm²) or lower. Therefore, itis preferable that the sprayed coating to be applied to the bearingportion of the pump has a number of the blow holes having a size largerthan 20 μm and present in the surface of 15 per square milimeter (mm²)or less, preferably 10 per square milimeter (mm²) or less. It is notedthat the WC - 12% cobalt sprayed coating and the Cr₃ C₂ - 25% NiCrsprayed coating as well as attained a similar phenomenon.

In order to evaluate the wear resistance of the bearing and the sleevehaving the sprayed coating subjected to the heat treatment against watercontaining earth and sand, the following element test was carried out.The method of the element test and a portion of results will now bedescribed with reference to FIGS. 18A-18D and 19. A rotation-sidespecimen 15 shown in FIG. 15A and a stationary-side specimen shown inFIG. 15B are dipped in water containing quartz sand and assumed to bewater containing earth and sand. Then, the rotary-side specimen isrotated as shown in FIG. 18D to be slid while being applied with apredetermined surface pressure. After they are slid for a predeterminedtime, the abrasion losses of the two specimens were measured. The wearlosses were evaluated by measuring the thickness of the rotation-sidespecimen 15 before and the after the test. The surface shape of thestationary-side specimen 16 before and after the test was measured by asurface roughness meter. The examining conditions are as follows: thesurface pressure is 1 to 10 kg/cm², the peripheral speed is about 0.5 to5 m/sec, and concentration of quartz sand is 0.1 to 10 wt. %. Thesliding test was repeated as follows: the water containing earth andsand was changed at each sliding operation covering a distance of 3.6 kmin order to reduce the influence of the wear of the quartz sand; and thequartz sand was interposed between the two specimens whenever watercontaining earth and sand was changed in order to cause the quartz sandto be easily caught between the two specimens.

FIG. 19 shows a portion of results of the experiments of the wearresistance of the sprayed coating subjected to the heat treatment. Theabscissa stands for the distance of sliding and the ordinate stands forthe average wear loss of the rotational side specimen. Thestationary-side specimen is α- SiC, while the rotation-side specimen isWC - 27% NiCr sprayed coating having a thickness of about 100 μm. Thebase is made of stainless steel of JIS SUS403 subjected to heattreatments of quenching and tempering. The conditions of the heattreatment applied to the sprayed coating was determined while referringto the results shown in FIG. 12 to be 400° C. for 20 hours. As acomparative material, a WC - 12% cobalt sintered article and non-treatedWC - 27% NiCr sprayed coating were used as the rotation-side specimensunder the similar conditions. The residual test conditions are asfollows: the surface pressure is 2 kg/cm², the peripheral speed is 0.5m/s and the concentration of the quartz sand is 9 wt. %.

Referring to FIG. 19, a line (1) designated by -Δ- shows the change inthe abrasion loss of the WC - 27% NiCr sprayed coating subjected to theheat treatment and applied to the bearing member according to thepresent invention, a line (2) designated by -o- shows the change in thewear loss of WC - 12% cobalt sintered article which is the comparativematerial applied to the conventional bearing member, and a line (3)designated by - Δ- shows the change in the wear loss of the WC - 27%NiCr sprayed coating which is the comparative material and to which noheat treatment is applied. The wear rate of the WC - 27% NiCr sprayedcoating which is not subjected to the heat treatment is about threetimes the wear loss of the WC - 12% cobalt sintered article. On theother hand, the wear rate of the WC - 27% NiCr sprayed coating issubstantially the same as the wear rate of the WC - 12% cobalt sinteredarticle. That is, the heat treatment set to 400° C. for twenty hourscauses the wear loss of the WC - 27% NiCr sprayed coating against watercontaining earth and sand was reduced to one-third. It can be consideredthat the foregoing reduction in the abrasion loss was caused from theincrease in the hardness due to the heat treatment shown in FIG. 12 towhich the sprayed coating was subjected. The wear resistance of thesprayed coating subjected to the heat treatment is a reasonable valuebecause of the relationship between the hardness of the sprayed coatingand the abrasion rate shown in FIG. 21.

Although omitted from FIG. 19, an excellent wear resistance is attainedfrom a combination of the WC - 27% NiCr sprayed coating subjected to theheat treatment set to 400° C. for 20 hours and Si3N4. The Si3N4 filmemployed by the fixed-side specimen exhibited excellent wear resistancesuperior to SiC.

Also the WC - 12% cobalt sprayed coating enables a similar effect to beobtained from a similar heat treatment as a result of a similarexperiment. However, the WC - 12% cobalt sprayed coating does not enablethe hardness to be improved unlikely the WC - 27% NiCr sprayed coating.Therefore, the wear resistance of the WC - 12% cobalt sprayed coating isinferior to that of the WC - 12% cobalt sintered article and that of theWC - 27% NiCr sprayed coating subjected to the heat treatment.Furthermore, cobalt employed as the binder material is inferior to NiCrin terms of the corrosion resistance. Therefore, there is a possibilityof generation of abrasion due to corrosion. Since the number of the blowholes in the surface of the aforesaid WC - 12% cobalt subjected to theheat treatment decreases, use of the cobalt binder material enables thewear resistance against corrosive abrasion to be improved. However, theattained wear resistance is insufficient against drain containingsalinity such as sea water. Therefore, a bearing which uses the WC - 12%cobalt sprayed coating cannot be used depending upon the conditions foruse.

As a result of a similar experiment, Cr₃ C₂ - 25% NiCr sprayed coatingenables improved wear resistance to be realized. However, the realizedimprovement in the wear resistance is inferior to that realized in theWC - 27% NiCr sprayed coating and WC - 12% cobalt sprayed coating. Thereason for this is that the fact that inherent hardness of the Cr₃ C₂ -25% NiCr sprayed coating is inferior to the hardness of the WC - 27%NiCr sprayed coating and that of WC - 12% cobalt sprayed coatingdeteriorates the effect of the heat treatment.

FIG. 20 collectively shows the wear rates of the results shown in FIG.19, results obtained when WC - 27% NiCr sprayed coatings are used inboth the stationary-side and the rotation-side specimens, and thoseobtained when the WC - 27% NiCr sprayed coating was used as therotation-side specimen and the Cr₃ C₂ - 25% NiCr sprayed coatingsubjected to the heat treatment was used as the stationary-sidespecimen.

FIG. 20 shows the wear rates of the stationary-side and therotation-side specimens after they were subjected to the experiment forsixty hours. The test conditions are the same as those of the experimentshown in FIG. 19. The relative wear rates shown on the axis of ordinateare values of the wear rates of specimens expressed by relative valueswhile making the wear rate of the rotation-side and the stationary-sidespecimens of (A) which is the combination of WC - 12% cobalt sinteredarticle and α- SiC to be a reference value 1.0. In the bar graph, thebars each having diagonal lines entered therein represent the wear ratesof the rotation-side specimens and the bars having no diagonal linesrepresent the wear rates of the fixed-side specimens.

The combination (A) is a combination of the WC - 12% cobalt sinteredarticle and α- SiC and is the results of a curve (2) shown in FIG. 19.Combination (B) is a result of a combination of WC - 27% NiCr sprayedcoating subjected to no heat treatment designated by curve (3) shown inFIG. 19 and α- SiC. Combination (C) is a result of a combination of theWC - 27% NiCr sprayed coating subjected to the heat treatment which isthe material of the bearing according to the present invention anddesignated by curve (1) shown in FIG. 19 and α- SiC. Combination (D) isa result of a combination of the WC - 27% NiCr sprayed coating subjectedto the heat treatment which is the material for the bearing according tothe present invention and the WC - 27% NiCr sprayed coating subjected tothe heat treatment. Combination (E) ms a result of a combination of theWC - 27% NiCr sprayed coating subjected to the heat treatment and theCr₃ C₂ - 25% NiCr sprayed coating subjected to the heat treatment. Itshould be noted that the conditions of the heat treatment to which theeach sprayed coating is subjected are as follows: 400° C. for 20 hours.

As described with reference to FIG. 19, the combination (C) enables bothof the stationary-side specimen and the rotation-side specimen to havethe wear resistance substantially equivalent to that attained fromcombination (A), that is, the combination of the WC - 12% cobaltsintered article and α- SiC. Also the combination (D) having thearrangement that the WC - 27% NiCr sprayed coatings subjected to theheat treatment are used in both of the stationary-side specimen and therotation-side specimen enabled the rotation-side specimen and thestationary-side specimen to have the wear resistance substantiallyequivalent to that of the WC - 12% cobalt sintered article.

The WC - 27% NiCr sprayed coating subjected to the heat treatment foruse in the stationary-side specimen exhibited excellent wear resistancesuperior to that of α- SiC. The hardness of α- SiC is an average valueof Hv 2704 (maximum value Hv 2874 and minimum value Hv 2591) which isconsiderably superior to the hardness of the WC - 27% NiCr sprayedcoating subjected to the heat treatment. The reason why the WC - 27%NiCr sprayed coating subjected to the heat treatment exhibited thesuperior wear resistance to that of a-SiC in spite of the aforesaid factlies in the difference in the ductility between the two materials, thatis, the difference in the fracture toughness. The fracture toughness ofα- SiC is about 3.9 (MN/m.√m) which is inferior to that of the WC - 27%NiCr sprayed coating, that is about 13 (MN/m.√m). It can be consideredthat the excellent fracture toughness of the WC - 27% NiCr sprayedcoating subjected to the heat treatment affects the wear resistancerather than the hardness does and the foregoing excellent wearresistance is obtained. It should be noted that the fracture toughnesswas measured by a Vickers impression method.

The combination (E) in which the WC - 27% NiCr sprayed coating subjectedto the heat treatment was used as the rotation-side specimen and the Cr₃C₂ - 25% NiCr sprayed coating subjected to the heat treatment was usedas the stationary-side specimen enabled the rotation-side specimen tohave a similar wear rate to that realized by each of the combinations(C) and (D). However, the wear rate of the Cr₃ C₂ - 25% NiCr sprayedcoating subjected to the heat treatment and used as the fixed-sidespecimen was higher than that of α- SiC and that of the WC - 27% NiCrsprayed coating subjected to the heat treatment. It can be consideredthat the reason for this lies in that the hardness of the Cr₃ C₂ - 25%NiCr sprayed coating cannot be significantly improved even if it issubjected to the heat treatment. However, the Cr₃ C₂ - 25% NiCr sprayedcoating exhibited equivalent or superior wear resistance to that of theWC - 27% NiCr sprayed coating if the temperature of the drain is high.It can be considered that the reason for this lies in that the hardnessof the Cr₃ C₂ - 25% NiCr sprayed coating does not deteriorateconsiderably at high temperature. Therefore, the combination of the WC -27% NiCr sprayed coating subjected to the heat treatment and the Cr₃C₂ - 25% NiCr sprayed coating subjected to the heat treatment causes asatisfactory effect to be obtained depending upon the condition for use.

If the WC - 27% NiCr sprayed coating subjected to the heat treatment isused in the sleeve to be combined with a bearing made of SiC or Si₃ N₄,excellent wear resistance and sliding characteristics can be obtained.In this case, it is preferable that SiC or Si3N4 be used in the bearinghaving a structure which is supported by the metallic backing member byshrinkage fit because the strength of SiC and Si3N4 is improved due tocompressive stress applied thereto. If the WC - 27% NiCr sprayed coatingsubjected to the heat treatment is used in both of the sleeve and thebearing, totally excellent wear resistance and sliding characteristicscan be attained. Although the WC sprayed coating containing cobalt asthe binder material as well as exhibits excellent wear resistance, it isdifficult to be used because of a problem of corrosion if drain containssalinity. If the WC - 27% NiCr sprayed coating and the Cr₃ C₂ - 25% NiCrsprayed coating subjected to the heat treatment are used as the sleeveand the bearing, the obtained wear resistance is inferior to thatobtainable from the combinations of the WC - 27% NiCr sprayed coatings.However, further improved characteristics can be realized if thetemperature of the drain is high. In this case, the WC - 27% NiCrsprayed coating may be employed in either of the sleeve or the bearing,resulting in the similar characteristics.

Therefore, the bearing structure according to the present inventionenables the contact surface of each of the sleeve and the bearing tohave similar wear resistance to that realized by a structure in whichWC - 12% cobalt sintered alloy is used as the bearing. Furthermore,assembling facility can be improved because the weight of the sleeve andbearing can be reduced. In addition, the fact that the sleeve and thebearing reveal excellent toughness will prevent fracture due to impact,causing the reliability to be improved. Therefore, a rotational machinesuch as a pump and a hydraulic turbine that incorporates the bearingstructure according to the present invention is attained satisfactoryreliability.

Although each of the foregoing embodiments employs the high speed flamespraying method for forming the sprayed coating, an explosive sprayingmethod, a pressure reduction plasma spraying method, a laser sprayingmethod or a plasma spraying method may be employed while freed fromparticular limitation. It should be noted that it is preferable that thehardness before the heat treatment is performed is higher. It ispreferable to employ the high speed flame spraying method or theexplosion spraying method because a hard sprayed coating can be formed.

Although each of the foregoing embodiments employs stainless steel ofJIS SUS403 as the material for the sleeve base and the bearing base,another material which can be used in water may be employed.Furthermore, it is preferred to employ stainless steel of JIS SUS403 inplace of stainless steel of JIS SUS304 because of a larger thermalcoefficient of expansion if the coefficient of thermal expansion of thesprayed coating, the main component of which is WC or Cr₃ C₂. Inaddition, it is preferable to employ stainless steel of JIS SUS304L orJIS SUS316L containing low carbon if the deterioration of the corrosionresistance of the stainless steel due to the heat treatment is takeninto consideration.

Although each of the foregoing embodiments employs NiCr and cobalt asthe binder material by contents of 12 wt. %, 25 wt. % and 27 wt. %, thecontent is not limited if the characteristics required for the sprayedcoating are met.

The present invention has the arrangement that the sprayed coating, themain component of which is WC or Cr₃ C₂, is heated under a predeterminedcondition. Therefore, the binding strength between the WC particles orCr₃ C₂ particles with the binder material is enlarged. As a result, theblow holes in the sprayed coating can be reduced, causing the average ofthe hardness to be increased by about 50 to 60%. Therefore, the wearresistance against water containing earth and sand can be improved bythree times the wear resistance before the heat treatment is performed.The thus realized characteristics are equivalent to those of theceramics sintered article such as the WC - 12% cobalt. Therefore, thesleeve and bearing formed by applying sprayed coating on the surface ofa metallic base can be used in place of the sleeve and the bearing madeof WC - 12% sintered article.

As a result, a bearing and a sleeve each having a large diameter can bemanufactured which cannot be manufactured by using the sintered articlessuch as WC, SiC or Si₃ N₄.

Furthermore, the weight of the bearing and that of the sleeve can bereduced regardless of the diameter, causing the assembling facility tobe improved significantly.

Moreover, stainless steel can be used to form the base in place of thesintered articles such as WC, SiC or Si₃ N₄ suffering fromunsatisfactory toughness. Therefore, the reliability of the bearing andthe sleeve can be improved. Consequently, a pump or a hydraulic turbineincorporating the bearing unit according to the present invention can beimproved.

Although the invention has been described in its preferred form with acertain degree of particularly, it is understood that the presentdisclosure of the preferred form has been changed in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A bearing unit comprising:a bearing; and a sleevewhich is in slide-contact with said bearing, wherein said bearing and/orsaid sleeve made of a metal having a contact surface applied with asprayed coating, the main component of which is WC, and which containsone or more elements selected from a group consisting of nickel,chromium and cobalt as a binder material thereof, and the number ratioof blow holes contained in the surface of said sprayed coating andhaving a size of not smaller than 20 μm is 15 per square millimeter orless.
 2. A bearing unit according to claim 1, wherein the thickness ofsaid sprayed coating is from 100 μm to 200 μm, and a contact portion ofsaid sleeve and/or said bearing is divided into two or more sections. 3.A bearing unit comprising:a bearing; and a sleeve which is inslide-contact with said bearing, wherein either of said bearing and saidsleeve made of a metal has a contact surface applied with a sprayedcoating, the main component of which is WC, and which contains one ormore elements selected from a group consisting of nickel, chromium andcobalt as a binder material thereof, and the residual is made of SiC orSi₃ N₄, or metal having a contact surface area applied with a sprayedcoating the main component of which is WC, and which contains one ormore elements selected from a group consisting of nickel, chromium andcobalt as a binder material thereof, and metal having a contact surfacearea applied with a sprayed coating the main component of which is Cr₃C₂, and which contains nickel and chromium as binder materials or metalwhich contains SiC or Si₃ N₄ and the number ratio of blow holescontained in the surface of said sprayed coating and having a size ofnot smaller than 20 μm is 15 per square millimeter or less.
 4. A bearingunit according to claim 3, wherein the thickness of said sprayed coatingis from 100 μm to 200 μm, and a contact portion of said sleeve and/orsaid bearing is divided into two or more sections.
 5. A bearing unitcomprising:a bearing; and a sleeve which is in contact with saidbearing, wherein said bearing and/or said sleeve made of metal has acontact surface applied with a sprayed coating, the main component ofwhich is WC, and which contains one or more elements selected from agroup consisting of nickel, chromium and cobalt as a binder materialthereof, and said sprayed coating is heated at a temperature of from300° C. to 550° C. after said sprayed coating has been formed, saidsprayed coating having a hardness of Hv 1000 or higher.
 6. A bearingunit according to claim 5, wherein the thickness of said sprayed coatingis from 100 μm to 200 μm, and a contact portion of said sleeve and/orsaid bearing is divided into two or more sections.
 7. A bearing unitcomprising:a bearing; and a sleeve which is in contact with saidbearing, wherein said sleeve is made of metal having a contact surfaceapplied with a sprayed coating, the main component of which is WC, andwhich contains one or more elements selected from a group consisting ofnickel, chromium and cobalt as a binder material thereof, and the ratioW/D² of weight W (kg) of said sleeve and the outer diameter D (cm) ofthe same is 0.05 or less.
 8. A bearing unit according to claim 7,wherein the thickness of said sprayed coating is from 100 μm to 200 μm,and a contact portion of said sleeve and/or said bearing is divided intotwo or more sections.
 9. A bearing member comprising a sprayed coatingformed on the surface thereof, wherein the number ratio of blow holescontained in the surface of said sprayed coating and having a size ofnot less than 20 μm is 15 pieces/mm² or less.
 10. A bearing memberaccording to claim 9, wherein said sprayed coating is mainly composed ofWC and contains one or more elements selected from a group consisting ofnickel, chromium and cobalt as a binder material thereof, or mainlycomposed of Cr₃ C₂ and contains nickel and chromium as a binder materialthereof.